Mössbauer studies of NaFes2 : magnetic hyperfine fields
and covalency in MFeS2 compounds (m=Na, k, Rb, Cs)
C.A. Taft
To cite this version:
C.A. Taft. Mössbauer studies of NaFes2 : magnetic hyperfine fields and covalency in
MFeS2 compounds (m=Na, k, Rb, Cs). Journal de Physique, 1977, 38 (9), pp.1161-1162.
<10.1051/jphys:019770038090116100>. <jpa-00208682>
HAL Id: jpa-00208682
https://hal.archives-ouvertes.fr/jpa-00208682
Submitted on 1 Jan 1977
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LE JOURNAL DE
TOME
PHYSIQUE
38,
SEPTEMBRE
1977,
1161
Classification
Physics Abstracts
76.80
MÖSSBAUER STUDIES
AND COVALENCY IN
NaFeS2 : MAGNETIC HYPERFINE FIELDS
MFeS2 COMPOUNDS (M=Na, K, Rb, Cs) (*)
OF
C. A. TAFT
Centro Brasileiro de
Pesquisas Fisicas, Av. Wenceslau Braz, 71,
Rio de
Janeiro, Brasil
(Reçu le 2 novembre 1976, révisé le 5 mai 1977, accepté le 23 mai 1977)
Résumé.
L’étude Mössbauer de NaFeS2, de la température ambiante à celle de l’hélium liquide,
révèle la présence d’ordre magnétique en dessous de 77 K. Le champ hyperfin à 4,2 K est de 270 kOe.
Les paramètres hyperfins du fer dans NaFeS2 indiquent la configuration à spin fort de Fe3+ (6S5/2),
Nous montrons que les valeurs réduites du champ hyperfin dans NaFeS2 comme dans les composés
MFeS2 (M=K, Rb, Cs) peuvent se corréler à l’aide de notions simples : champ hyperfin transféré
par les électrons 4s, électronégativité des cations.
2014
Abstract.
Mössbauer studies of NaFeS2 from room temperature down to liquid helium temperature indicate a magnetic ordering below 77 K. The magnitude of the hyperfine field is 270 kOe
at 4.2 K. The measured hyperfine parameters characterize iron in NaFeS2 to be in the high
spin Fe3+ 6S5/2 configuration. We show that in NaFeS2 2014 as in the other MFeS2 compounds
(M=K, Rb, Cs) - the reduced values of hyperfine fields (150-270 kOe) can be correlated with simple
concepts such as transferred hyperfine fields via 4s contribution and cationic electronegativity.
2014
In our previous work we report1. Introduction.
ed on Mossbauer studies of covalency effects in
alcali-dithioferrates AFeS2 (A=K, Rb, Cs). These
effects cause a large reduction of the observed magnetic
hyperfine fields from the expected ionic value [1, 2].
In this work I report on Mossbauer studies of
NaFeS2 in the temperature range (300-4.4 K) indicating also a large deviation of the measured magnetic
field from the ionic value.
It is interesting to note that the magnitude of the
magnetic field in the alcali-dithioferrate series can be
correlated with a covalency scale based on 4s contribution and cationic electronegativity.
-
The samples were investigated
1024 multichannel analyser, operating in the
multiscaler mode to a velocity transducer of the
Kankeleit type. The source was a 10 mCi of " Co
in a Pd matrix having a width of 0.28 mm/s measured
against a natural iron absorber. The gamma rays were
detected with a thin Nal (Tl) scintillator mounted
in a photo-multiplier tube. The liquid helium measurement was taken with both source and absorber in
liquid helium. The low temperature measurements
were recorded using an Elscint cryostat with tempe2.
using
Experimental.
-
a
(*) This paper was presented at the International Conference
the Applications of the Mossbauer Effect, Corfu (Greece) 1976.
on
rature controller.
The temperature stability was
The
2
°C.
±
samples were prepared by fusing iron
with
sodium carbonate and sulphur and
powder
the
cold
leaching
product in water.
3. Results and discussion.
Figure 1 shows the
transmission spectra of NaFeS2 at several temperatures. From 300 K down.to 77 K a quadrupole
doublet with a temperature independent splitting of
0.58 mm/s is observed. The room temperature isomer
shift value is 0.36 mm/s (w.r.t. Fe). These parameters
are typical for iron in the high spin Fe3+ 6S5J2 configuration [1-4]. At 4.4 K the spectrum was additionally
split by a strong magnetic hyperfine interaction
and the magnitude of the hyperfine field was
I Hhf 270 k0152 ( ± 5 kOe). The sign of Hhf is
assumed to be negative as this is usually the case
in Fe3+ high-spin compounds.
Electric field gradient (EFG) calculations [1, 2]
in this series of compounds indicated negligible values
of asymmetry parameter (q). In the present case of
a small quadrupole interaction superimposed on a
large magnetic hyperfine interaction in the absorber,
the observation of an unchanged quadrupole splitting (1) indicates that the main axis of the EFG coin-
=
(’) (= shift of the inner lines
of the six-line pattern).
pairs
versus
Article published online by EDP Sciences and available at http://dx.doi.org/10.1051/jphys:019770038090116100
the outermost lines
,
1162
one
Hhf
=
obtains Q4s = 0.22. This estimate yields
330 k0e, which is a reasonably good esti-
mate.
Various attempts have been made to correlate the
extensive Mossbauer data with available electronegativity scales [5]. In this isostructural series MFeS2
(M=Na, K, Rb, Cs) in which the cation M is the
only varying element, it appears reasonable to attempt
such a correlation. The increasing electronegativity
of the cation will decrease the 4s electron density at
the Fe nucleus and hence reduce the proposed transferred hyperfine fields. One would thus expect the
compounds with the larger electronegativity values
to be more ionic, i.e., have larger values of hyperfine
fields. The fields measured at 4.4 K are considered
as saturation values (for Na and CsFeS2) and as
negative. In figure 2, I have plotted the measured
FIG. 1.
-
Absorption spectra
for
NaFeS2
at various
temperatures.
cides with the axis of the magnetic hyperfine field.
In this case a negative sign of the EFG can be simply
derived.
The magnitude of the internal field in NaFeS2
(270 k0e) which is assumed to be the saturation field,
is unusually small for a ferric ion. There are apparently considerable covalency effects from neighboring spins. Molecular orbital calculations suggest
that there should be a linear relationship between
the covalent induced hyperfine field and the charge
transfer to the 4s state. Assuming such a relationship we can write [1]
FIG. 2.
-
Magnetic hyperfine fields in MFeS2 (M K, Rb, Cs, Na)
vs cationic (M) electronegativity.
=
magnetic field vs cationic electronegatiwhich
vity [6, 7],
suggests that the cationic electroneis
gativity effectively a good covalency parameter in
this series of compounds.
values of
where Hionic = - 630 kOe is the ionic field and
+ 1 348 kG is the average hyperfine field
H4s
due to one 4s electron and Q4, is the number of
4s electrons at the Fe3 + ion originating from the
covalent ligands. From the Modified WWJ Plot [1]
=
I am grateful
Acknowledgments.
for
financial
CNPq (Brasil)
support.
-
to OEA and
References
[1] TAFT, C. A., RAJ, D. and DANON, J., J. Physique Colloq. 35
(1974) C 6-241.
[2] TAFT, C. A., RAJ, D. and DANON, J., J. Phys. &#x26; Chem. Solids 36
(1975)283.
[3] TAFT, C. A., RAJ, D. and DANON, J., Phys. Stat. Sol. 64 (1974)
[4]
111.
TAFT, C. A., Proceedings of the International Conference
Mössbauer Spectroscopy, Cracow, Poland (1975).
[5] GONSER, U., Ed. Mössbauer Spectroscopy (Springer-Verlag,
Berlin Heidelberg, New York) 1967.
[6] PRITCHARD, H. O. and SKINNER, H. H., Chem. Rev. 55 (1955)
745.
[7] PAULING, L., The
Nature of the Chemical Bond
Press, 3rd Ed., New York) 1960.
on
(Cornell Univ.